This application is based on Japanese Patent Application No. 11-239364 filed Aug. 26, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a piston for a swash plate type compressor, and more particularly to a die-cast piston for such a compressor, and a method of producing such a piston by die-casting.
2. Discussion of Related Art
A piston for a swash plate type compressor is conventionally produced by forging or die-casting. Described more specifically, a blank for the piston is formed by forging or die-casting, and the blank is subjected to appropriate machining operations, to produce the desired piston. The swash plate type compressor piston may be a single-headed piston or a double-headed piston. The single-headed piston includes a head portion and a neck portion which are slidably movable in a cylinder bore formed in a cylinder block of the compressor. The neck portion is a generally U-shaped portion having a base section and a pair of substantially parallel arm sections which extend from the base section. The double-headed piston includes two head portions on the opposite sides of the neck portion. Since the single- or double-headed piston is reciprocated within the cylinder bore, it is generally required to reduce the weight of the piston. For this reason, there has been proposed a piston formed of an aluminum alloy and designed to have a wall thickness as small as possible. On the other hand, the base section of the neck portion of the piston is subject to repeated application of a bending moment during a reciprocating movement of the piston. To assure the intended durability of the piston, therefore, the reduction of the wall thickness is limited. While the piston produced from a forged blank has a comparatively high degree of strength, the piston by produced by die-casing inevitably has a lower strength.
It is therefore a first object of the present invention to provide a die-cast piston for a swash plate type compressor, which piston has a sufficiently high degree of durability while having a reduced weight. A second object if the invention is to provide a method of producing such a lightweight, highly durable die-cast piston.
The first or second object may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.
(1) A die-cast piston for a swash plate type compressor including a cylinder block having a cylinder bore formed therein, the piston comprising a generally cylindrical head portion slidably movably received in the cylinder bore, and generally U-shaped neck portion having a base section and a pair of substantially parallel arm sections which extend from the base section, the die-cast piston being characterized in that the base section has an inner surface including at least one as-cast surface area formed in a die-casting process.
In the die-cast piston constructed according to the above mode (1) of this invention wherein the inner surface of the base section of the neck portion includes an as-cast surface area or areas, the base section has an increased degree of durability or strength. Generally, a die-cast article or product has a chilled layer adjacent to its surface, and the chilled layer left unmachined after the die casting process is effective to improve the strength of the die-cast article. The chilled layer is formed by rapid cooling and solidification of a molten mass of iron in a casting mold, at a portion of the molten mass in contact with the inner surface of the mold which defines a mold cavity. The chilled layer is characterized by a discontinuous change in the crystallization ratio of the primary crystal or xcex1-phase (proeutectic) and the eutectic silicon with respect to each other. Since the chilled layer has high values of hardness and strength, the presence of the chilled layer adjacent to the inner surface of the base section of the neck portion is effective to increase the bending strength of the base section, particularly, the durability of the base section. The inner surface of the base section, which is located close to the outer circumferential surface of the swash plate of the compressor, is conventionally subjected to a machining operation, which results in the removal of the chilled layer. In the piston according to the present invention, however, the chilled layer adjacent to at least a portion of the inner surface of the base section is left unmachined, so as to provide at least one as-cast surface area, so that, like a forged piston, the present die-cast piston has sufficiently high degrees of durability and strength owing to the chilled layer while having a significantly reduced weight. Although the inner surface of the base section is preferably left as-cast over a surface area as large as possible, the inner surface may be required to be more or less machined at some area, for the purpose of removing a rib or casting fins. A die-cast blank that is subjected to machining and other working operations to produce the piston is usually provided with a reinforcing rib or ribs to assure accurate and efficient machining of the blank. The casing fins are inevitably formed on the blank, at a parting plane of the mold at which the mold halves are butted together to define the mold cavity. Where the inner surface of the base section is partially machined, the surface area to be machined is desirably minimized to maximize the total as-cast surface area for maximizing the durability and strength of the base section of the neck portion.
(2) A die-cast piston according to the above mode (1), wherein the inner surface includes a central machined surface area and a pair of as-cast surface areas, the central machined surface area being located at an intermediate portion of the inner surface as viewed in a direction perpendicular to a centerline of the piston which passes a center of the generally cylindrical head portion, the central machined surface area being formed as a result of a machining operation in the intermediate portion of the inner surface, and wherein the pair of as-cast surface areas are located on opposite sides of the central machined surface area as viewed in the direction.
The die-cast blank which is processed to produce the die-cast piston is generally provided with reinforcing rib or ribs for the purpose of preventing thermal strains of the blank during heat treatment thereof and reducing elastic deformation during machining operations on the blank. For instance, a rib is formed so as to extend between a pair of arm sections which extend from a base section of a neck portion of the blank in a direction perpendicular to the centerline of the blank which passes the center of the head portion, so that the rib connects the base section and the two arm sections. The rib is formed so as to extend from a central portion of the inner surface of the base section, which central portion is located an intermediate portion of the inner surface as viewed in the direction perpendicular to the centerline. The rib is eventually removed by a machining operation, so that the die-cast piston does not have the rib. Accordingly, the central portion of the inner surface of the base section of the blank must be subjected to a machining operation to remove the rib, so that the inner surface of the base section inevitably has a central machined surface area. In other words, the inner surface of the base section of the piston can have a pair of as-caset surface areas on the opposite sides of the central machine surface area as viewed in the direction in which the arm sections extend. The as-cast surface areas left on the inner surface of the base section are effective to increase the neck portion of the piston.
(3) A die-cast piston according to the above mode (1), wherein a substantially entire portion of the inner surface is an as-cast surface area.
The piston according to the above mode (3) is most preferred from the standpoint of the durability of the neck portion.
(4) A die-cast piston according to any one of the above modes (1)-(3), wherein the pair of arm sections have opposed inner surfaces having as-cast surface areas adjacent to the at least one as-cast surface area of the base section, the as-cast areas of the arm sections being also formed in the die-casting process.
The opposed inner surfaces if the pair of arm sections are usually machined to be flat surfaces in which part-spherical recesses are formed by machining so that part-spherical shoes are partially received in the respective recesses, for sliding contact with the opposite surfaces of the swash plate of the compressor. For increasing the durability of the neck portion, the opposed inner surfaces of the arm sections preferably remain as-cast, particularly, at the end portions of the opposed inner surfaces adjacent to the end portions of the inner surface of the base section. Generally, there are provided fillets of a small radius of curvature at the boundaries of the inner surface of the base section and the opposed inner surfaces of the arm sections, in order to reduce the stress concentration at those boundaries. For increasing the durability of the neck portion, the fillets are preferably left as-cast, namely, preferably have as-cast surfaces.
(5) A die-cast piston according to any one of the above modes (1)-(4), wherein the head portion includes a body section having a circular shape in transverse cross section and cooperating with the cylinder bore of the compressor to partially define a pressurizing chamber, the piston further comprising a connecting portion which connects the head portion and the neck portion.
The configuration of the die-cast piston according to the above mode (5) is desirable for facilitating the removal of the blank from the casting mold. Usually, the casting mold consists of two mold halves which are butted together so as to define a parting plane which includes the centerline of the head portion having the circular body section and which is parallel to the direction of extension of the arm sections from the base section. Where the rib indicated above is not formed so as to extend from the intermediate or central portion of the inner surface of the base section, the casting fins are likely to be formed on this central portion due to the abutting contact of the two mold halves at the parting line indicated above. In this case wherein the rib is not formed, the central portion is preferably subjected to a machining operation to remove the fins. As in the case wherein the inner surface of the base section is machined to remove the rib, the machined surface area is desirably minimized to maximize the as-cast surface area, in the case where the inner surface of the base section is defined.
(6) A die-cast piston according to the above mode (5), wherein the head portion further includes a sliding section extending from the body section and connecting the body section and the connecting portion.
The sliding section provided between the circular body section of the head portion and the connecting portion is effective to assure smooth sliding movement of the piston within the cylinder bore, without an inclination of the centerline with respect to the centerline of the cylinder bore. However, the sliding section desirably has a relatively small weight to reduce the overall weight of the piston. To reduce the weight of the sliding section, the sliding section preferably consists of a radially outer sliding section and a radially inner sliding section which correspond to radially outer and inner portions of the cylinder block.
(7) A method of producing a die-cast piston for a swash plate type compressor including a cylinder block having a cylinder bore formed therein, the die-cast piston comprising a generally cylindrical head portion slidably movably received in the cylinder bore, and a generally U-shaped neck portion having a base section and a pair of substantially parallel arm sections which extend from the base section, the method comprising the steps of:
forming a blank by die casting such that the blank includes: a head portion which gives the head portion of the piston; a neck portion which gives the neck portion of the piston and which includes a base section and a pair of arm sections; and a reinforcing portion which extends so as to connect the pair of arm sections of the neck portion of the blank, in a direction parallel to a centerline of the blank which passes a center of the head portion of the blank; and
subjecting the blank to a machining operation to remove the reinforcing portion such that the base section of the neck portion of the blank has an inner surface including at least one as-cast surface area which is left unmachined.
Conventionally, the inner surface of the base section of the neck portion of the blank which give the inner surface of the base section of the neck portion of the piston is machined upon removal of the reinforcing portion. In the method according to the above mode (7) of the present invention, the area of the inner surface of the blank to be machined is minimized to maximize the as-cast surface area, for the purpose of maximizing the durability and strength of the neck portion of the piston produced by processing the blank.
(8) A method according to the above mode (7), wherein the reinforcing portion is a rib extending from a central portion of the inner surface of the base section of the blank in a direction of extension of the pair of arm sections of the blank, the central portion being located at an intermediate portion of the inner surface of the base section of the blank as viewed in a direction perpendicular to the centerline, the rib connecting the base section and pair of arm sections of the blank, and wherein the step of subjecting the blank to a machining operation comprises removing the rib such that the inner surface of the base section of the blank includes a central machined surface area and a pair of as-cast surface areas located on the opposite sides of the central machined surface area as viewed in the direction perpendicular to the centerline.
In the method according to the above mode (8), the central portion of the inner surface of the base section of the neck portion of the blank is removed when the rib is removed by the machining operation. However, there are left as-cast surface areas in the portions of the inner surface located on the opposite sides of the central machine surface area formed by the removal of the rib. These as-cast surface areas are effective to increase the durability of the neck portion of the piston produced from the blank.
(9) A method according to the above mode (7), wherein the reinforcing portion is a bridge portion extending to connect the pair of arm sections of the blank in the direction parallel to the centerline, the bridge portion being spaced apart from the inner surface of the base section of the blank, and wherein the subjecting central portion being located at an intermediate portion of the inner surface of the base section of the blank as viewed in a direction perpendicular to the centerline, and wherein the step of subjecting the blank to a machining operation comprises removing the bridge portion and machining a central portion of the inner surface of the base section of the blank to remove fins formed in the step of forming a blank, such that the inner surface of the base section of aid blank includes a central machined surface area and a pair of as-cast surface areas located on the opposite sides of the central machined surface area as viewed in the direction perpendicular to the centerline.
In the method according to the above mode (9), the fins formed along the parting plane of the mold halves are removed by machining the central portion of the inner surface of the base section of the neck section of the blank, such that the portions on the opposite sides of the central machined surface area are left as as-cast surface areas, which improve the durability of the neck portion of the piston produced by processing the blank.
(10) A method according to any one of the above modes (7)-(9), wherein the step of subjecting the blank to a machining operation comprises:
positioning a rotary cutting tool having a peripheral cutting edge such that the rotary cutting tool is rotatable about an axis which is parallel to the centerline and which is aligned with a center of the reinforcing portion as viewed in the direction perpendicular to the centerline;
feeding the rotary cutting tool in a radial direction thereof toward the inner surface of the base section of the neck portion of the blank while the rotary cutting tool is rotated, so that the reinforcing portion extending between the pair of arm sections of the blank is removed.
In the method according to the above mode (10) wherein the rotary cutting tool is used to remove the reinforcing portion, the portions of the inner surface of the base section of the neck portion of the blank located on the opposite sides of the central machined area can be easily left as the as-cast surface areas.
(11) A method according to any one of the above modes (7)-(10), wherein the pair of arm sections of the neck portion of the blank have respective opposed inner surfaces which are opposed to each other in the direction parallel to the centerline, and the rotary cutting edge further has side cutting edges formed on opposite sides of the peripheral cutting edge, and wherein the machining operation comprises machining at least a portion of each of the opposed inner surfaces such that an end portion of the each of the opposed inner surfaces which is adjacent to the inner surface of the base section of the blank is left as an as-cast surface area.
In the method according to the above mode (11) wherein the machining operation is performed by the rotary cutting tool having both the peripheral cutting edge and the side cutting edges, not only the central portion of the inner surface of the base section of the blank but also portions of the opposed inner surfaces of the pair of arm sections are removed by the same rotary cutting tool. To produce the piston, two recesses are formed in the above-indicated machined portions of the opposed inner surfaces of the arm sections, so that shoes are partially received in the recesses, for sliding contact with the opposite surfaces of the swash plate of the compressor. The machining operation on the opposed inner surfaces is performed such that the end portions of the opposed inner surfaces adjacent to the inner surface of the base section are left as cast, in order to reduce the stress concentration at the boundaries between the inner surface of the base section and the opposed inner surfaces of the arm sections, which stress concentration would easily take place in the absence of the as-cast surfaces at those boundaries. Thus, the durability of the piston produced from the blank is further increased according to the above mode (11) of this invention.